The present invention relates to an improved process for the oxidation of methyl-substituted aromatic compounds. More particularly, the present invention comprises an improved liquid phase catalyst system for the preparation of aromatic aldehydes, especially p-hydroxybenzaldehyde.
In U.S. Pat. No. 4,113,782, formylated phenoxy compounds are prepared by air oxidation of methylated phenoxy compounds such as p-methoxytoluene in the liquid phase in the presence of co-catalysts comprising lower fatty acids or anhydrides and at least one soluble salt of cobalt, manganese, chromium or nickel.
In EP No. 12,939 to Sumitomo Chemical Company Ltd., p-cresol is oxidized in the liquid phase by an oxygen-containing gas in the presence of base and a cobalt compound or metallic cobalt. Cobalt porphyrin complex was disclosed as a suitable catalyst at page 7, line 4. Amines were taught to be suitable solvents, however, later research has indicated that amines in general are not suitable solvents.